The electrical discharge machining art has advanced from the early stages in which relaxation oscillators were used to provide machining power pulses. Independently timed and controlled pulse generators are now almost universally used and in these generators electronic switches are generally employed in the form of solid state switches or banks of parallel connected switches, particularly transistors. In the electrical discharge machining process, sometimes hereinafter referred to as "EDM", it is necessary that as the workpiece material is removed a predetermined gap be maintained between the tool electrodes and the workpiece through an automatic servo-feed system which provides a continuous advance into and toward the workpiece as the material removal progresses. During the electrical discharge machining process, a fluid coolant, generally a liquid, is circulated through the machining gap to flush the removed workpiece particles from the gap. The coolant is usually furnished under pressure by a pump through one or more openings provided in the electrode and workpiece. One necessary and defining characteristic of electrical discharge machining is that the coolant is a dielectric fluid, such as kerosene, transformer oil, distilled water or the like. The dielectric fluid is broken down in minute, localized areas by the action of the machining power pulses passed between the closely opposed surfaces of the tool electrode and workpiece and having its spacing known as the "EDM machining gap". For control of the servo-feed system, there is generally utilized an electrical signal from the machining gap in order to control the rate and the direction of servo-feed. In many cases, this gap signal is compared to an adjustable reference voltage so that the operator can select the rate of servo-feed desired for the particular machining condition.
It will thus be seen that with respect to the servo-feed of the gap elements in electrical discharge machining a parameter of the gap, whether it be average gap voltage as shown and described in Williams U.S. Pat. No. 2,841,686 issued July 1, 1958, peak gap voltage as shown and described in Webb Reissue 25,542 issued Mar. 24, 1964, or as in any other EDM servo-feed systems, there is required a reliable signal from the gap. The problem has been complicated by the fact that for certain electrode materials, such as graphite, the normal EDM gap polarity in which the tool electrode is negative and the workpiece positive must be reversed to provide for best machining results. Additional problems are encountered when the same power supply circuit is used to trigger the output switches of several different machining gaps, that is, where several spaced and insulated electrodes are used to simultaneously cut several holes in the same workpiece or, alternately, several electrodes are used to cut holes at the same time in different workpieces. As power supply circuits have come to require more power capability, lead lengths in order to handle the multiple gap systems have become longer and it has become continuously more difficult to provide for gap sensing and to provide for those circuit connections which will be immune to noise in the electrical system and provide reliable voltage signals to control servo-feed.
Other function control circuits are likewise dependent in their operation upon the magnitude of electrical signals derived from the machining gap, which signals again operate as an index to control, for example, interruption of power when a gap short circuit condition has occurred. The gap short circuit condition most often occurs due to a build-up of the eroded materials from both the electrode tool and workpiece to provide a bridging of the machining gap. Unless this condition is rapidly alleviated, either by electrode back-up by instantaneous interruption of power to the gap, serious damage can result to either or both the electrode tool and the workpiece. This problem has been solved by the prior art by the development of a number of gap short circuit protection systems such as that disclosed and claimed in Sennowitz U.S. Pat. No. 3,539,145 issued on Apr. 15, 1969 for "EDM Power Supply Circuit". The need of a reliable sensing system responsive to gap voltage to control such a protection system is particularly met by the system according to the present invention.
An additional problem arises from the failure of one or more of the electronic output switches, generally with their power conducting electrodes in a shorted condition. Power to the gap must be interrupted until the defective transistor can be located and replaced. The problem again is one of providing a reliable sensing circuit capable of responding to such a condition and providing an output signal to provide the necessary protection function control.
In some gap short circuit protection systems there is provided not a total interruption of the machining power but rather a control of the individual machining power pulses themselves such as by narrowing their on-time or increasing the off-time, or in some cases performing both these functions together. An example of an on-off time control system for controlling pulse width is disclosed in the aforementioned U.S. Pat. No. 3,825,713 of which the present application is a continuation-in-part.
By way of summary, it will be seen that the prior art protection systems of electrical discharge machining apparatus have the common problem of providing a control signal whether it be from the machining gap itself or from an intermediate part of the power supply such as the electronic output switch, which signal represents an abnormal or malfunctioning condition and actuates a protection circuit to suitably respond either by way of interrupting the power to the gap, reducing the power to the gap, or initiating a servo-feed back-up operation to protect the gap elements from possible damage.
It will be understood in the specification that when I refer to "electronic switch" I mean any electronic control device having several electrodes comprising at least two principal or power conducting electrodes acting to control current flow in the power circuit, the conductivity between the principal electrodes generally being controlled by a control electrode within the switch whereby the conductivity of the power circuit is controlled statically or electrically without movement of mechanical elements within the switch. Included within the definition are transistors in which turn-on is accomplished by a control voltage applied to the transistor control electrode and in which turn-off is accomplished automatically in response to removal of that control voltage. Also included in the definition are devices of the gate type in which turn-on is accomplished by a control voltage applied to the control electrode, which control voltage may be then removed and in which turn-off is accomplished by application of a subsequent control voltage to the control electrode. An additional class of electronic switches, called "electronic trigger devices", falls within this definition and includes thyratrons, semi-conductor controlled rectifiers and the like. By "electronic trigger device" I mean any electronic switch of the type which is triggered on at its control electrode by a pulse and is turned off by reverse voltage applied for a sufficient time across its principal electrodes.